Both were new 5 months (4000 km) ago. The chain is about 60% worn/stretched. I've been riding 5 days/week, 40 km/day, Toronto on mild winter roads, and using only 'dry' lube (i.e. alcohol and teflon) on the chain.

What should I know about that?

I like to keep my bike in good working order, so that it rides very well (but, arriving home each evening after an hour-long commute, I have had no time for bike care: I just lock it, in the laundry room). Is this rust only cosmetic?

Should I use a winter lube? What (product or servicing) should affect/prevent rust on the cassette, if that's necessary?

When I replace the chain (when it's next serviced, perhaps two months from now) should I ask the LBS to replace the cassette again as well?

4 Answers
4

It's unclear how much rust you're talking about, but it's unlikely that a bike could develop serious rust on a chain in 5 months unless left outside with no maintenance.

I've dealt with some pretty rusty bikes over the years (especially when I spent a spell working on donated bikes for Christmas Anonymous) and I've never seen a cog that was so rusty that it was unserviceable.

The chain, of course, is a different matter, but generally rust on a chain is of little consequence unless it causes "stiff links", and that won't happen on a bike that's used regularly.

I'd recommend that you switch to a slightly less "dry" lube (or at least give the chain a few squirts of regular oil every now and again). Also, if you've been cleaning the chain at all, be a little less diligent cleaning it -- it's not necessary to get it shiny bright, you just want to get off the crud.

Lube
In winter time i use a special heavy duty lube. It doesn't protect the chain a lot better than other oils but it wears off much slower giving you a bit of slack on the maintenance. None of this matters of corse if you don't clean it every now and then. If you clean it once a week, say every friday before locking it away for the weekend, you should be fine.

Cassette
Even with proper maintenance/cleaning it's hard to completely avoid rust in the winter. The road salt loves the metal on your bike as much as you do. A narrow steel wire brush is really good at removing surface rust.

When it comes to chains: They are cheap. Better to change it once too often and save the more expensive cassette so you can ride it longer.

As @Daniel R Hicks, In my experience chains can show surface rust very quickly once having regular exposure to water, especially in the winter if roads are salted. However, don't worry too much by the rust.

If you are cleaning your chain using water & degreaser etc. then apply water repellent such as WD40 to the chain afterwards. Wipe dry and then lube.

If riding regularly in wet weather I'd as above recommend a specific wet lube, typical application is put a reasonable amount on and wipe off excess. Move the chain up and down the rear cassette to give that a coating.

In dry weather I use a dry lube as they attract less muck but it is best to use a "little and often" approach to adding to your running gear.

Re. Cassette:
It is advisable to change your rear cassette at the same time as your chain so that you get even wear, otherwise the shifting can be affected.

I expect to change the chain about every 6 months (5000 km). Is that the right frequency for changing the rear cassette at the same time, or should the cassette be more like every 12 months (10000 km)?
–
ChrisWFeb 21 '12 at 16:20

1

As stated elsewhere, if you change your chain when it's worn then you should get 2-4 chains per cluster. It's only if you let the chain go too long that replacing the cluster with the chain becomes necessary.
–
Daniel R HicksFeb 21 '12 at 23:47

The chain is not ruled by the calendar or by mileage/kilometerage; it is ruled by elongation and/or by corrosion damage. Chain life varies depending on chain type, maintenance, riding conditions, and strength and weight of the rider. However, as a ballpark number, figure on replacing the chain every 1,000-1,500 miles (1600-2400km) if the bike is ridden in dirty conditions or with infrequent lubrication by a heavy rider. Lighter cyclists riding mostly on clean, dry roads can extend the replacement time to 2,000-3,000 miles (3218-4828km) with poor maintenance, and up to 5,000 miles (8046km) with a daily high-quality lubrication.

The best thing you can do to improve chain life and performance is to frequently wipe it down and lubricate with a high quality chain lube* appropriate for the conditions. (This is basically ~10 minutes every day. Park, wipe, lube, done.)

The next thing is to regularly check for elongation, say a couple of times a month. You can do this with a chain-elongation gauge or with a ruler. When it becomes elongated; replace it. You don't replace the chain by the calendar.

Your chainrings and cogs:

The number one thing determining the lifespan of your chainrings and cogs is how well you maintain your chain!

If you always replace the chain when necessary (elongated), and keep it clean/lubed you can easily go through 3 or more chains before needing to replace the cogs.

Cleaning the cogs, chainrings, and the rest of the drive train frequently (at least weekly) will also expand the lifespans of these components. (This takes 10 to 15 minutes for a quick cleaning.)

In your situation, your cog-set may be damaged at this point, and if so, you need to replace it. However, once you replace that and your chain, you can get a lot more mileage out of your entire drivetrain with some basic, regular maintenance and lubrication. With a quick daily wipe-down and lubrication, the rust issue should disappear.